My 2 month old baby has some kind of rash on her kneck and white stuff from it, what could it be?

Need to see doctor. It's hard to diagnose based on your description. Possibilities include bacterial skin infection and yeast rash, so i'd advise seeing her doctor, as each cause is treated differently. I hope it clears up soon!
Yeast infection? Make an appointment with her doctor, your baby may have a yeast infection on her neck. This is common but does require medication. It could also be a bacterial infection but not as likely. Keeping her neck area dry and clean helps prevent this from reoccuring once treated.